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What's the meaning of "sample rate" in MP3/Vorbis/AAC?, Considering the audio was transformed
sheh
post Oct 7 2012, 03:44
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What's the meaning of "sample rate" in MP3/Vorbis/AAC? Wouldn't it be meaningless after transform, same as bit depth?

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[JAZ]
post Oct 9 2012, 19:03
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Mmm.. the tranformation that (most) lossy codecs do from time domain (samples) to frequency domain (frequency bands intensity and phase) does not compress by itself. It might even need more data, depending on the precision.

What codecs use to reduce the bitrate demands is allowing frequencies to be less precise (quantizing the different possible values), coupled with other compressing techniques (joint stereo with less bits for side channel, huffmann compression, parametric audio reconstruction, etc..).

But going back, what you usually get from the transformation is not "frequency 1Hz, x intensity, frequency 2Hz y intensity,...). To get that, you would need to use an FFT of the same size of the sampling rate, and in that case, you would need to say which size it is. (effectively defining the sample rate).

Generally, though, a fixed size (depending on sample rate, to have enough definition) transformation is used. In MP3, it is an overlapped window of 1152 samples (in case of long blocks) (Please, correct me if i am wrong!), which generates 576 frequency bands (and their phases). Those 576 values by themselves don't mean a thing, because there is the same number of bands from a 32Khz wave than from a 48Khz wave.

Concretely, the band 576, for a 32Khz contains frequency information from the frequency 31943Hz to the frequency 31999Hz.
For a 48Khz, that same band contains frequency information from the frequency 47915Hz to the frequency 47999Hz.
It is only on playback that the frequency gets any meaning. And to understand it, you might think on what happens when playing a 22Khz file at 44Khz, or playing an LP of 33RPM at 45RPM.

This post has been edited by [JAZ]: Oct 9 2012, 19:06
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- sheh   What's the meaning of "sample rate" in MP3/Vorbis/AAC?   Oct 7 2012, 03:44
- - saratoga   No, the transform occurs on sampled data and so th...   Oct 7 2012, 05:05
|- - sheh   I guess the way I put it does seem like a retread....   Oct 9 2012, 05:21
|- - saratoga   QUOTE (sheh @ Oct 9 2012, 00:21) Benski s...   Oct 9 2012, 19:03
- - pdq   Still. since the data are stored as frequencies an...   Oct 7 2012, 14:23
- - [JAZ]   When we say that lossy streams don't have a st...   Oct 7 2012, 14:44
- - jensend   I'm not intimately acquainted with the details...   Oct 7 2012, 16:46
- - benski   fs is part of the MDCT equation. If you didn...   Oct 7 2012, 18:32
|- - sheh   Okay, so the original fs needs to be plugged in so...   Oct 8 2012, 18:33
- - lvqcl   Winamp MP3 decoder still have Full/Half/Quarter qu...   Oct 7 2012, 18:56
|- - benski   QUOTE (lvqcl @ Oct 7 2012, 13:56) Winamp ...   Oct 10 2012, 16:16
- - saratoga   Using a dct means you could easily drop the sample...   Oct 7 2012, 19:00
|- - saratoga   QUOTE (sheh @ Oct 8 2012, 13:33) Okay, so...   Oct 8 2012, 23:41
- - jensend   I still think you're wrong here. Once you...   Oct 7 2012, 19:57
|- - saratoga   QUOTE (jensend @ Oct 7 2012, 14:57) I sti...   Oct 7 2012, 20:16
- - Dynamic   I think one problem is that we're never doing ...   Oct 8 2012, 23:04
- - [JAZ]   Mmm.. the tranformation that (most) lossy codecs d...   Oct 9 2012, 19:03
|- - Dynamic   QUOTE ([JAZ] @ Oct 9 2012, 19:03)...   Oct 9 2012, 22:22
- - pdq   So am I understanding correctly that 99.9% of deco...   Oct 10 2012, 17:12
|- - saratoga   QUOTE (benski @ Oct 10 2012, 11:16) QUOTE...   Oct 10 2012, 17:30
- - [JAZ]   QUOTE (Dynamic @ Oct 9 2012, 23:22) As [J...   Oct 10 2012, 17:52
|- - saratoga   QUOTE ([JAZ] @ Oct 10 2012, 12:52...   Oct 10 2012, 18:20
- - [JAZ]   Winamp with the in-house decoder (i.e. not the fra...   Oct 10 2012, 18:41
|- - saratoga   QUOTE ([JAZ] @ Oct 10 2012, 13:41...   Oct 10 2012, 18:55
- - Dynamic   My recollection is that Winamp 1.7 running under W...   Oct 11 2012, 16:14
- - sheh   So sample rate matters. I suppose without being fa...   Oct 11 2012, 23:20


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